The type of media (+ or -) shouldn't make any difference. The brand of media and quality of it will make a BIG difference. With the limited info you are giving its hard to tell. The 1st copy may be ok to play but not of good enough quality to copy again. You can actually get this problem with original purchased dvds. It could be from the quality of the blank media you used the first time. If you are burning the 2nd copy faster than 4x you may want to drop your burn speed down a notch. Use some well known blanks like Ritek or Verbatim grade A. Don't try to run a copy through a ripping software again, just use a straight disc copy program. Do not multi task while burning. Just a few suggestions to try. Hope they help.

what do you mean run a straight copy? I have done a straight copy using dvd decrypter and burn with dvd shirnk (altho i havent re-encoded at all or changed any settings..)
Have also tried a straight *.iso copy using dvd decrypter and that didnt even load on the dvd player..